The technology relates to a method of forming a layered coating film. In particular, the technology relates to a method of forming a layered coating film by means of intercoat-less coating in coating of a vehicle such as, but not limited to, an automobile.
In coating of a vehicle body such as, but not limited to, an automobile body, a layered coating film has been formed that may include an electrodeposition coating layer, an intercoat layer, a topcoat layer, and a clear layer. The electrodeposition layer may have corrosion resistance. The intercoat layer may have light beam blocking properties, and the topcoat layer may have design properties. The clear layer may be provided for surface protection. These layers may be sequentially layered on a steel sheet that may serve as a workpiece.
Recently, a method (so-called intercoat-less coating) has been adopted in which the intercoat layer is eliminated from the layered coating film as mentioned above, and the topcoat layer is formed directly on the electrodeposition coating layer. This method eliminates one layer from the layers of the existing layered coating film, resulting in a shortened coating procedure as well as elimination of a coating material used for the intercoat layer. This method is therefore significantly advantageous in terms of productivity and costs. Also, elimination of the intercoat layer contributes to weight reduction of a vehicle body, and advantageously leads to enhanced fuel consumption performance and CO2 reduction.
On the other hand, since the intercoat-less coating provides little light beam blocking that ought to be performed by the intercoat layer, light beams may pass through to reach the electrodeposition coating layer. This may cause deterioration in weather resistance of the electrodeposition coating layer, resulting in possibility of exfoliation at an interface between the electrodeposition coating layer and the topcoat layer.
In the intercoat-less coating, it is therefore desirable to impart the topcoat layer with the light beam blocking properties that have been originally imparted to the intercoat layer. Accordingly, in general, an ultraviolet absorber may be added to the topcoat layer. With regard to a wavelength region (300 nm to 420 nm both inclusive) that involves difficulties in light beam blocking by the ultraviolet absorber, possible measures may be to add a dark and deep colored pigment, e.g., carbon black, or to increase a film thickness of the topcoat layer.
However, the addition of the dark and deep colored pigment or the increase in the film thickness may cause concerns such as darkness of an appearance of the layered coating film and lowered design properties.
Japanese Unexamined Patent Application Publication (JP-A) No. 2000-70850 discloses a technique to cope with such lowered design properties. Specifically, an electrodeposition coating layer, a solid color layer, and a clear layer are sequentially formed on a steel sheet. The solid color layer includes fine particles of zinc oxide as an ultraviolet absorber. The fine particles of zinc oxide absorb light beams in wavelength regions of not only 300 nm to 360 nm both inclusive but also 360 nm to 420 nm both inclusive. This allows for elimination of the addition of the dark and deep colored pigment, e.g., carbon black, or elimination of the increase in the film thickness, attaining improved design properties.